Modular integrated semisubmersible

ABSTRACT

A semisubmersible floating offshore structure. The hull is defined by individual modular buoyant columns. A separate buoyant pontoon is provided at the lower end of each column. Topside structural framing between the columns retains the columns in their spaced relationship. A truss section is attached to the pontoons and a heave plate is attached to the lower portion of the truss section.

FIELD AND BACKGROUND OF INVENTION

The invention is generally related to offshore structures and more particularly to semisubmersible offshore structures.

For a floating structure like a semisubmersible, heave is governed by draft and the geometry of the platform. A common method to reduce heave is by increasing draft so that the first-order motion due to the wave loads can be reduced. However, increasing draft can reduce the advantage of quayside topside integration due to the limiting capacity of lifting cranes. This means that there are limitations that apply in choosing the option of increased draft when quayside topside integration is intended to be a preferred solution.

Another method to reduce heave and maintain a shallow draft is by reducing pontoon vertical area. By doing so, the first order motions due to the wave loads can also be reduced. Payload capacity, however, may not be achieved due to lack of displaced volume from the platform. In addition, the heave natural period may be close to the peak period of the wave energy due to lack of added mass from the pontoon. Consequently, dynamic amplification of heave motions will be higher.

Besides quayside topside integration, the semisubmersible has another advantage in terms of cancellation loads between pontoon and column in heave. In potential theory, the first order loads on the pontoon and column come from the Froude-Kriloff force and the diffraction force. These forces are either in-phase or 180 out-of-phase with undisturbed wave elevation at the center of the semisubmersible. At a certain frequency, they cancel each other.

FIG. 1 shows a typical heave force component of a semisubmersible with a ring pontoon at the bottom of the column. Inertia and drag loads in the pontoon and column vary with wave periods. At a period of 6 and 22 seconds, the inertia loads from the column cancel out the inertia loads from the pontoon. In other words, the summation of forces acting on the pontoon and column are equal to zero at periods of 6 seconds and 22 seconds, assuming that the drag load contribution is small at these periods. For this reason, the RAO's (response amplitude operators) at periods of 6 and 22 seconds are also zero. FIG. 2 shows the corresponding heave RAO from the semisubmersible.

The first rise of the heave RAO (period between 6 and 22 seconds) comes from the imbalance of inertia loads between the pontoon and column. In this case the pontoon load is larger than that of the column. The peak of the second riser (periods above 22 seconds) is the natural period of the system and it is governed by mass and added mass of the system, spring stiffness (waterplane area, mooring, and risers) and damping coefficients.

Since the peak of wave energy, especially in hurricane conditions, normally occurs at the first hump of the heave RAO, a new floating structure is required so that the first hump is low enough that the heave of the floating structure can accommodate dry tree risers. The relative size of the column and pontoon can be optimized to minimize the total vertical hydrodynamic load on the hull, thereby reducing the heave.

SUMMARY OF INVENTION

The present invention addresses the shortcomings in the known art. The invention provides a semisubmersible floating structure that has a hull defined by individual modular buoyant columns. A separate buoyant pontoon is provided at the lower end of each column. Topside structural framing between the columns retains the columns in their horizontal and vertical spaced relationship. A truss section is attached to the pontoons and a heave plate is attached to the lower portion of the truss section.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a part of this specification and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same:

FIG. 1 is a graph that illustrates the heave force components of a typical semisubmersible structure.

FIG. 2 is graph that illustrates the heave RAO of a typical semisubmersible structure.

FIG. 3 is a perspective view of the invention.

FIG. 4 is a side view of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is generally indicated in FIG. 3-4 by numeral 10. The modular semisubmersible structure 10 is generally comprised of columns 12, a topside framework of deck framing 14 and optional braces 15, pontoons 16, truss frame section 18, and heave plate 20.

The columns 12 are retained in their horizontally spaced relationship from each other by a topside framework that is formed of a deck framing 14 and optional braces 15. A separate pontoon 16 is provided at the lower end of each column 12. The truss frame section 18 is a space frame that is attached to the pontoons 16 and extends downwardly therefrom. The heave plate 20 is attached to the lower end of the truss frame section 18.

The columns 12 and pontoons 16 are modular and can be constructed in a conventional yard separately from the rest of the structure. The columns may have any cross sectional shape such as round or square as desired for a particular situation to provide acceptable motion performance, stability, buoyancy, and/or storage. Also, while FIG. 3 illustrates the presence of four columns 12, it should be understood that this is for illustrative purposes only and the number of columns 12 used for the structure may be three or more. The columns may be arranged in a number of patterns including but not limited to a square, rectangle, pentagon, hexagon or an octagon. The number of columns increases as payload warrants. The pontoons 16 are illustrated in FIG. 3 as being round in cross section. However, this should not be considered as a limitation on the configuration of the pontoons 16. As with the columns 12, the pontoons may be round, square, or even a plurality of smaller buoyant tanks positioned around and attached to each column 12.

The deck framing 14 and optional braces 15 are attached to the columns 12 and maintain the horizontal and vertical spacing between the columns 12.

The truss frame section 18 is an open space frame that is attached to each pontoon 16 and extends downwardly therefrom. The number of legs 22 that define the truss frame section 18 is determined by the number of columns 12 and pontoons 16. As seen in FIG. 4, the legs of the truss frame section are connected together by horizontal braces 24 and X-braces 26.

The heave plate 20 is attached to the lower end of the truss frame section 18 so as to be horizontal when the structure is in its operational position. The heave plate 20 is provided with a center well 28 to accommodate drilling and/or production risers. The deck framing 14 is also provided with a center well 30 that is aligned with the center well 28. The heave plate 20 serves to cancel vertical loads from the columns 12. The heave plate 20 may be a rigid plate or a soft tank that can be used to adjust ballast or buoyancy of the structure. Because the location of the heave plate is far from the mean water line, the inertia loads are reduced so that the net inertia force between the columns and the heave plate is minimized. The size and draft of the heave plate, as well as the columns 12, should be designed such that the first rise of heave RAO is at an acceptable level for use of dry trees on the structure. The heave plate also adds mass to the structure such that the heave natural period of the modular semisubmersible is away from the peak period of the wave energy.

During construction of the invention, the columns 12 and pontoons 16 may be constructed at a conventional yard away from the topsides, deck framing 14, optional braces 15, truss frame section 18, and heave plate 20. The columns and pontoons may be wet towed or transported dry on a vessel to a yard for assembly of the structure.

The invention provides several advantages over the prior art.

The design makes it possible to use dry tree risers and preserves the motion advantage of the spar and quayside topsides integration advantage of the semisubmersible structures without increasing the cost associated with steel weight, fabrication methods, pre-service procedures (tow and installation), and operation maintenance.

The invention provides an assembly that can be fabricated in a conventional yard, which is a result of the configuration and dimensions of the modular columns. The modular columns can also be fabricated in a different yard from that of assembly.

There is no traditional pontoon connected to the columns that is the main source of inertia loads in the vertical direction as discussed earlier and shown in FIG. 1. The buoyancy required for floatability of the structure is achieved partially from the bottom of the column modules.

The invention provides a structure having relatively low vertical loads. Because the invention has relatively low vertical loads, the column draft does not need to be deep.

Because the heave plate is far from MWL (Mean Water Level), the inertia loads are reduced so that the net inertia force between the modules and the heave plate is minimized.

The invention is construction friendly. The pontoon is built on land and is moved to the quayside using a crane. The column is built on top of the pontoon. One completed modular column is ready to be wet towed or dry towed using a vessel/barge to an assembly yard. In addition, the full structure including heave plates, truss, pontoons, columns, and topsides can be assembled in a dry dock.

The invention has certain advantages over a spar type structure. The heave period can be designed to be above thirty seconds. More tensioner type top tensioned risers (TTRs) can be accommodated. Typically, the spar heave period is below 28 seconds. For a spar structure, in order to avoid heave resonance due to tensioner stiffness contributions, the number of heave plates needs to be increased thereby increasing the overall length of the spar hull and therefore one-piece hull transportation may not be possible. The more TTRs, the more likely heave resonance will occur. With new metocean criteria (post hurricane Katrina) in the Gulf of Mexico, the spar heave motion may have significant impact. The invention does not require upending at the offshore installation site for topside installation. Hook up and commissioning of the invention can be done at quayside.

Typical dimensions for a structure built according to the invention would be as follows. It would have a draft of 280 feet, a height of 340 feet from the heave plate to the top of the pontoons, 50 foot diameter columns, 80 foot diameter pontoons, a 160 foot tall truss section, with a heave plate that is 200 feet square and approximately 10 feet high.

The invention provides advantages over other dry tree semisubmersible concepts. The first rise in the RAO of the invention is lower and could have a significant benefit on riser fatigue life and riser tensioner stroke. The truss and heave plate/soft tank are proven concepts. For the Gulf of Mexico, the modular column operating draft of the invention can be made below one hundred twenty feet, which has significant advantages in terms of hull and topside integration at quayside.

To prove the feasibility of the invention as a dry tree floating semisubmersible, a preliminary analysis of motion responses has been made using WAMIT and HARP applications. The heave RAO of the invention is better than that of a typical dry tree semisubmersible even though it has a deeper draft than the invention. This improvement is due to the geometry of the modular columns and pontoon.

While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles. 

1. A semisubmersible floating offshore structure, comprising: a. a plurality of columns; b. a framework attached to said columns; c. a separate pontoon attached to the lower end of each column; and d. a truss framework attached to the pontoons and extending downward therefrom.
 2. The structure of claim 1, further comprising a heave plate attached to the lower end of said truss framework.
 3. The structure of claim 1, wherein said columns provide buoyancy to the structure.
 4. The structure of claim 1, wherein said heave plate is a soft tank.
 5. A semisubmersible floating offshore structure, comprising: a. a plurality of buoyant columns; b. a framework attached to said columns; c. a separate pontoon attached to the lower end of each column;. d. a truss framework attached to the pontoons and extending downward therefrom; and e. a heave plate attached to the lower end of said truss framework.
 6. The structure of claim 5, wherein said heave plate is a soft tank. 